The present invention relates to a method of forming a solder joint between metal layers and, more specifically, to a method of bonding a semiconductor chip to another semiconductor chip or a circuit board using a solder joint between metal layers.
Three-dimensional (3-D) and 2.5-dimensional (2.5-D) packaging is a technique facilitating higher bandwidths and shorter wiring lengths and is critical to realizing performance improvements in future computing systems. In 3-D and 2.5-D packaging, the joint terminal pitch and bump size are dramatically smaller than those in conventional flip chip packaging. As a result, faults due to stress on joints and their interfaces and faults due to electromigration (EM) caused by rising current densities (assuming a constant current) have become an issue.
In micro-joints, the joint (structure) is usually composed of a copper (Cu) pillar and solder cap in order to maintain a gap between substrates and joints for underfill (UF) and to prevent inter-solder shorting. The copper pillar is used to disperse the current flowing into the solder joint.
Generally speaking, there are two types of joints (structures). In the first type, all of the solder in the joint is an intermetallic compound (IMC). Here, two substrates with joints are pressed together using only weight control, or, as taught in Japanese Patent Application No. 2014-04198, the substrates are stored at a high temperature for a long time after they have been joined together. In the other type of joint, solder remains in the joint. Here, height control is added using reflow or a flip chip bonder (second joining method).
In the first joining method, the solder joint is made to be almost entirely an intermetallic compound and is therefore EM resistant, but stress is concentrated because of its shallow thickness. In the case of the second joining method, some of the solder has lower EM resistance than the first joining method so it still experiences problems with low EM resistance.
Therefore, improvement of EM resistance has been attempted using a structure in which the entire joint is made of an intermetallic compound (IMC) but enough thickness to suppress joint stress has been retained. For example, in a full IMC method in which a thick joint is made entirely of an intermetallic compound, the sample may be heated at a high temperature after joining. However, when a resin is present, such as in the pre-applied resin technique used in fine pitch applications, long-term, high-temperature heating methods experience problems from the standpoint of resin degradation.